| Abstract |
- Organic matter dynamics in riparian ecosystems are largely driven by interactions among hydrology, soil, and vegetation. In two riparian meadows, northeast Oregon, I examined the hypothesis that vegetation and soil characteristics in three plant communities - defined as wet, moist, and dry meadow - were strongly influenced by hydrological and redox variables associated with the geomorphological position of each community on the floodplain. Along short transects that extended from stream-side wet communities to terrace dry communities, I sampled plant species composition, biomass, and soil; calculated carbon and nitrogen pools; and monitored water table elevation, soil redox potential, and dissolved organic carbon (DOC) and nitrate-nitrogen (nitrate-N) in shallow ground water. Strong gradients in water table elevation and soil redox potential existed along the transects.. Water table elevation followed seasonal patterns of stream stage, and was consistently highest in wet communities, intermediate in moist communities, and lowest in dry communities. Soil redox potential indicated seasonal anaerobic conditions ([less than or equal to] 300 mV) in the wet and moist communities, and year-round aerobic conditions in the dry communities. Plant communities differed markedly in species richness (dry> moist> wet), total biomass (wet> moist> dry), and ratios of belowground-to-aboveground biomass (wet> moist> dry). Soil carbon and nitrogen pools were highest in dry communities and similar in wet and moist communities. Vegetation and soil characteristics were strongly correlated to median water table elevation and redox potential. Ecosystem (biomass + soil) carbon ranged from 7.01 to 11.7 kg/m², with [approximately] 2 to 4 % in aboveground biomass, [approximately] 2 to 23% in belowground biomass, and [approximately] 81 to 95% in soil. Ecosystem nitrogen ranged from 0.035 to 0.093 kg/m², with [approximately] 0.7 to 1.6 % in aboveground biomass, [approximately] 1.5 to 6 % in belowground biomass, and [approximately] 94 to 98% in soil. In shallow ground water, concentrations of DOC and nitrate-N were two to six times greater than in stream water, with highest values in wet communities. Collectively, these results indicate that, in unconstrained reaches, water table elevation and soil redox potential strongly influence distribution of plant species, and dynamics and storage of organic matter.
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